A circuit configuration which supplies a signal obtained via a gas sensor that is a measure of the lambda air factor in the exhaust gas of combustion processes is described in the technological book “Ottomotor-Management/Bosch” [Bosch Gasoline Engine Management] Vieweg Publishers, 1st edition 1998, pages 22-23. The gas sensor has a pump cell situated between a first electrode and a second electrode. The first electrode is exposed to the exhaust gas. The second electrode is situated in a measuring gas space which is acted upon by the exhaust gas via a diffusion barrier. The second electrode together with a third electrode situated in a reference gas space forms a Nernst cell which supplies an almost abrupt voltage change in the area of a stoichiometric gas mixture present at the second electrode of the Nernst cell. The reference gas space is filled with oxygen as the reference gas. The required oxygen concentration in the reference gas space is adjustable using a reference pump current, which is supplied to the third electrode in the reference gas space. The reference pump current results in an oxygen ion transport out of the measuring gas space of the Nernst cell into the reference gas space.
A difference amplifier compares the voltage occurring at the Nernst cell with a reference voltage, which is set at approximately 450 mV, corresponding approximately to the middle of the voltage jump of the Nernst cell. The difference amplifier sets a pump cell pump current which is supplied to the first electrode. The pump cell pump current results in an oxygen ion transport in the pump cell which attempts to keep the oxygen content in the measuring gas space at a level at which a stoichiometric mixture prevails. The pump cell pump current may be used as the output signal of the gas sensor. It corresponds to a measure of the lambda air factor in the exhaust gas.
The oxygen concentration in the reference gas space varies constantly due to diffusion processes. Oxygen ions are diffusing out of the reference gas space while hydrocarbons are diffusing into the reference gas space. The two processes result in an unwanted decline in the oxygen concentration, which is known as poisoning of the reference gas space.
German Published Patent Application No. 43 33 231 describes a method for operating an oxygen sensor having an internal pumped reference in which the reference pump current is increased temporarily under certain operating conditions to eliminate any poisoning that might already be present or to prevent imminent poisoning.
An unwanted change in the oxygen concentration in the reference gas space may also be caused by a distortion of the reference pump current due to leakage currents which may occur between the feeder line to the third electrode and feeder lines to a sensor heater, for example.
An object of the present invention is to provide a circuit configuration which permits reliable operation of a gas sensor having a reference gas space.